1. Field of the Invention
This invention relates to purification of chemical compounds and, in particular, the purification of compounds related to optical fiber manufacture.
1. Art Background
The compounds SiCl.sub.4 and GeCl.sub.4 are commonly used in the preparation of glass bodies such as optical fibers. In the production of these bodies, the chloride glass former SiCl.sub.4 is reacted with oxygen to form silicon oxides and the chloride glass former GeCl.sub.4 is reacted with oxygen to form the dopant GeO.sub.2. (In operation the chlorides are converted to oxides which then form the glass. However, for pedagogic purposes SiCl.sub.4 and GeCl.sub.4 are termed chloride glass formers.) The silicon oxides thus are doped with a material such as germanium dioxide that produces a desired refractive index configuration in the glass body being formed from the SiCl.sub.4. At one time or another, a plethora of impurities has been reported to be present in chloride glass forming compounds. However, small amounts of SiHCl.sub.3, for example, present in SiCl.sub.4 have been identified as producing a disproportionate degree of degradation to the transmission properties of the final fiber. Trichlorosilane is also generally the impurity occurring in the greatest quantities, e.g., at concentration levels that are hundreds of times greater than other impurities. Therefore, purification efforts have been directed to the removal of the impurity, SiHCl.sub.3 that is considered to degrade significantly the properties of optical fibers. It has been found that fibers made using SiCl.sub.4 which contains 200 ppm or more of SiHCl.sub.3 significantly attenuate signals in several regions of the advantageous wavelength region between 0.9 and 2.5 .mu.m that are guided by the fibers. This attenuation results from the absorption of OH moieties incorporated into the fibers through the presence during fiber production of impurities having available hydrogen.
Various methods have been developed to remove trace amounts of SiHCl.sub.3. For example, conventional distillation of SiCl.sub.4 has been employed to remove this impurity. (See U.S.S.R. Patent Specification and Inventor's Certificate by I. D. Spevak et al published Apr. 25, 1976, Bulletin No. 15, Application No. (21)1453563/23-26). The use of photochlorination to remove SiHCl.sub.3 has been described. (See Offenlegungsschrift No. 2805824.) This technique is based on the well-known reactivity of Si--H bonds with halogens. Chlorine gas is dissolved in the chloride glass former that is to be purified and the glass former is introduced into a region that is illuminated by light having a wavelength of about 240 to 400 nm. The chlorine is induced by the light to react with SiHCl.sub.3 to produce SiCl.sub.4 and HCl. Since this photochlorination process is extremely rapid, short treatment times are possible and are utilized to achieve high throughput.